Polypropylene and wax coating formulation



r i Patented July 3, 1962 i 2 v 3 042 640 is important because of thestrain to which the bottle is y 9 subjected in handling. .Blockingshould be. avoided as POLYPROPYLENE AND WAX COATING o FORMULATION wallas motthn and a tendency toward scuffing, found John J. Gersic,Elmhurst, and Lawrence B. Nelson, Garden City, N.Y., assignors to SoconyMobil Oil Company, Inc, a corporation of New York No Drawing. Filed Jan.20, 1960, Ser. No. 3,478

8 Claims. (Cl. 26028.5)

This invention is directed to a wax formulation for coating paper,cardboard, cartons and the like and is particularly concerned with acoating composition containing a substantial proportion of petroleumwax.

Many items have been packaged in paper and cartons in recent years whichwere formerly transported in bottles and boxes. In order to provide aleakproof container, the package is coated with a layer of paraiin wax.While substantial volumes of paraifin wax have been used for thispurpose, certain defects in the wax coated container have annoyed themanufacturer and user of these packages. Continuousefi'ort has been madethrough the years to improve the wax formulation by refinery treatmentor additions to the wax or both and considerable improvement in thecoating wax has been made. And yet in spite of this progress furtherimprovement is desired and will have to be made to prevent competingtypes of packaging formulations from being developed and used asa'replacement for the wax coating.

For bread wraps and paper sealed food containers,

the wax formulation must be transparent to show printed matter, capableof being used as a heat seal and must not flow even when packaged hot.The wax formulation must show considerable ductility even at lowtemperature and high tensile strength. For coating milk containers thewax formulation must provide low temperature ductility and yet provide ahard surface even at elevated temperatures. The composition must notfiake in the milk container and must not scuff on shelves or tables whenrubbed across such surfaces. Wax processing techniques have beendeveloped to provide the correct amount of non-normals in the paraffinwax,'thereby providing improved ductility. Oil has been added to the waxin some instances to improve ductility and removed from the wax in otherinstances to improve hardness and scuff resistance. line Wax has beenadded to the paraflin to improve ductility and raise the tensilestrength as well as the melting point. Microcrystalline wax has beendivided into a plastic wax component and a hard ceresin wax component.The plastic wax has been added to prow'de better sealing in bread wrapformulations. The ceresin wax has been added to reduce scutiing andprovide frictional feel in dairy wax formulations. Polyethylene has beenadded in varying molecular weights and amounts to provide variousimprovements.

For instance, US. Patent No. 2,031,036 discloses a paraffin Wax mixedwith 20% microcrystalline wax and a suspended colloid of natural andartificial resins, modified phenol-formaldehyde resins and othercondensation and polymerization products on the market. This compositionis shown to be useful for coating paper, card.- board or other material.The 'U.S. Patent No. 2,298,846 discloses the addition of plasticmaterials to wax to provide improved coating compositions for paper andsheet materials used to protect food studs. The plastics arepolyalkylenes such as polyethylene, polybutylene, etc. The suggestedwaxes are crystalline parafiin, amorphous or microcrystalline,petrolatum, and high melting point waxes like carnauba wax.

Also, US. Patent No. 2,638,459 states that the tensile strength ofparaifin wax used for coating paper bottles Microcrystalparticularly inparaflin wax, should be avoided. The patentees point out that the use ofpolyethylene has not been practical because the high molecular weightsolid ethylene polymers dissolve with difiiculty in molten parafiin waxeven at the ISO- F. normally used for waxing machine operations. Thepatentees state further that while solid ethylene polymers in meltedcondition are miscible with paraffins in all proportions, they tend tocrystallize from solution at temperatures below about 230 F. The patentteaches that very small amounts of ethylene polymers having a molecularweight of 5,00025,000 can be added to parafiin or microcrystalline Waxand are eifective in improving the wax.

And further, US. Patent No. 2,728,735 discloses a coating wax formed bymixing paratfin wax, ceresin or high-melting type paraffin wax obtainedfrom petrolatum stock, and a polymerized olefin having a molecularweight greater than about 1,000 such as polyethylene of 1,000 100,000molecular weight.

As indicated in the prior artdiscussion presented above, parafiin waxesand wax formulations are widely used as coating for paper and othermaterials. They form hard, glossy coatings that are good barriers tomoisture vapor and to aqueous liquids. The largest single use is in thecoating of dairy and juice containers. Other uses include frozen anddried food packaging. As indicated, however, the paraffin wax coatingsare brittle and have an annoying tendency to crack and flake oif underimpact or other stress. While the ductility of paraffin waxes may beincreased by additions of waxy isoparafiins, microcrystalline waxes, andlubricating oils, this improvement is obtained only at the sacrifice ofother essential properties. The coatings are too soft or they rub off onclothing. They may feel greasy instead of being hard, glossy andplastic-like. This makes the container difiicult to grasp. It iscustomary, therefore, to add only a limited amount of additive materialfor ductility improvement in order not to excessively debase otherrequired properties.

We have discovered coating wax formulations possessing excellentductility without debasement of other prop erties. a parafiin wax, aminor amount of microcrystalliue or ceresin wax, and a critical amountof polymers of propylene alone or in combination with mineral oil.

' The object of this invention is to provide an improved coating wax forapplication to paper and paperboard.

A furtherobject of this invention is to provide an improved dairycoating wax formulation for application to milk cartons of paperboard.

A further object of this invention is to provide an improved coating waxfor dip coating milk cartons which provides resistance to flaking andresistance to rub-off, has a non-greasy feel, provides improved handlingand has exceptional ductility.

These and other objects of the invention will be more fully disclosed inthe following detailed description of the invention.

Waxed dairy cartons are coated by a dipping procedure in which thepreformed carton is immersed in a bath of molten wax and then allowed todrain to remove the excess wax. The resultant wax coating, whensolidified, froms a glossy, liquid-resistant film on the paper cartonwhich may be used to store milk, orange juice and other liquids. Thesecontainers are either coated at a central location and shipped toindividual dairies for filling or are formed, dipped and filled at thedairy. The formed carton is dipped into the wax bath at a temperaturebetween ISO-180 F. The proper These formulations comprise a major amountof proper thickness.

' bles to form in the coating.

- weight resinous or solid propylene polymer.

' period is critical in that if it is too short, an excessive amount ofwax will be used, and if it is too long, 'tO'O much wax will drain fromthe container walls and leave dry areas which are not protected withwax. Since the dipping and draining cycles are usually fixed, the temfperature of the wax bath is regulated so that the wax has the properviscosity and will deposit a coating of'the One of the problemsencountered in the past in choosing additives and components to add to'the base dairy wax has been the Viscosity of the re- 205 F., excessiveamounts of the residual moisture from the-paperboard are expelled,causing pinholes and bub These bubbles'result in a lack of protectionthat particular area and the coating is unsuitable. Also, at theseexcessive temperatures, enough moisture is expelled from the paperduring the dipping operation to cause it to become brittle. Thus, anyblend prepared for dip coating dairy cartons must have a viscosity belowa predetermined maximum level.

Most parafiin waxesused for coating purposes have a ductilities of 0.01to 0.02 inch. 'Ductility is measured on an inclined: plane as isdisclosed in Modern Packag- TABLE I Ductility Grams Wax (ElongationFlaked ofi in Inches) Per 1,000

' Cartons We have found that wax compositions suitable for coatingpurposes can be obtained by the following combinations:

I V (1) Parafiin wax, with or without microcrystalline wax orceresiniwax and a small amount of a liquid propylene polymer of lowmolecular weight. v

(2) Paraffin wax, microcrystalline or ceresin wax or their. combination,and a mixture of a liquid propylene polymer of low molecular weight witha higher molecular The resinous polypropylene may be either isotactic,made by the polymerization of propylene with a stereo-specific catalyst;or it may be atactic, made by a random polymerization process. V V V (3)Same as (2), above, except that an atactic propylene resin is used whichis produced during the polymerization of isotactic polypropylene andseparated therefrom by solvent extraction and precipitation.

(4) Paraflin wax, microcrystalline wax or ceresin wax, and a mixture ofatactic propylene resin and a mineral oil.. We have found that onlytheatactic propylene resin will cooperate with a mineral oil to yield waxcompositions of high ductility.

, The parafiin waxes used inthis invention are the nor- 4 mallyavailable grades-of coating wax having melting points of about 120150 F.The oil content of these waxes may vary between about 0-3 although weprefer waxes with oil contents below 1% by weight.

Conventional microcrystalline wax is obtained from heavy distillate oilsor residual lubricating oils by We known solvent precipitationprocedures. The wax is obtained from the solvent solution by cooling theliquid to a temperature range of about 40-60" F., producing a wax whichmelts at about 150-170 F. The conventional microcrystalline wax .may beconsidered to be made up of twocomp'onents, a ceresin wax componentmelting at high temperature and a plastic'wax component melting at anintermediate temperature. The ceresin wax component is composedpredominantly of normal and slightly branched parafi'ins. The lowermelting plastic wax component is composed of highly branched and cyclichydrocarbons. Microcrystalline waxes have melting points in the range ofISO-185 F. whereas the ceresin waxes usually melt at about 185- 200 F.We prefer to add ceresin wax to microcrystalline wax in our formulationsor use ceresin wax in place of microcrystalline The ceresin wax impartsbetter appearance to the wax formulation and improved the feel orbandleability of the coating.

Theliquid propylene polymer oil can be obtained by polymerizingpropylene in chloroform solution with a suspended aluminum halidecatalyst. The molecular weight of the oil may.vary from about 400-1500.The liquid polymer maybe used as prepared or'may be bydrogenated. Inboth instances the results were equally satisfactory.

The propylene polymer oil is prepared by polymerizing propylene ineither hexane or chloroform using about 3% of suspended anhydrousaluminum chloride as a catalyst. Propylene at 12 p.s.i.g. is introducedinto the stirred mixture which is kept at 110-120" F. until the reactionis complete, as indicated by the failure of the system to absorb morepropylene. After the reaction mixture is allowed to settle and thealuminum chloride sludge drawn'off, the liquid polymer is washed with'water (5 gal./ 100 lbs.) and then with an equal amount of 5% sodiumcarbonate. This mixture is then stripped of solvent at atmosphericpressure and 300 F., followed by a vacuum stripping at 26 inches of Hgvacuum, and

finally a filtration with Hyflo (trade name) filter aid.

Typical properties of the product oil prepared by the proceduredescribed hereinabove'are:

Color Yellow to straw.

Molecular weight 670 to 850. Viscosity- Cs. F. 584 to 2200. Cs. 210 F. a23.5 to 57.

The product oil contained about 0.80 to; 0.95 double bonds per molecule.

Isotactic propylene resin, made with stereospecific catalyst is nowcommercially available. For example, the isotactic propylene resinmanufactured and sold by the Hercules Powder Company under the tradename Pro-Fax may be used. Alternatively, the propylene resinmanufactured and sold by Montecatini Soc., Gen, Milan, Italy, under thetrade name Moplen may also be used. The molecular weight ofthis-material may vary from about 100,0005 00,000. V i

Atactic polypropylene is of random structure and not specificallyoriented. This resin may-have a molecular weight of about 5,00010'0,000.A variety of nonspecific catalysts are available for preparation ofatactic propylene resin and are well known to workers in this field. Theatactic resin can also be obtained as a byproduct of the manufacture ofisotactic polypropylene.

The mineral oil used in certain formulations with the atacticv propyleneresin is a 100-200 second (viscosity at 100 F. S.U.S.) commerciallyavailable U.S;P. grade oil. U.S.P. grade oils are not essential unlessthe coating is likely to come in contact with edible materials.

Example I A 125/ 127 AMP commercial parafiin wax was subjected to theinclined plane ductility test and provided a ductility (elongation:inches) of 0.018.

Example 11 A 133/ 135 AMP commercial paraffin wax was subjected to theductility test and provided a ductility of 0.013 inch.

Example III A ceresin wax having a melting point of 185 F. (ASTMdesignation: Dl27-49) was subjected to the ductility test and yielded aductility of 0.013 inch.

Example IV A blend of 80% 125/127 AMP paraffin wax and 20% ceresin waxof melting point 185 F. was subjected to the ductility test and showed aductility of 0.022 inch.

Example V A blend of 85% 125/ 127 AMP paratlin wax and 15%microcrystalline wax of melting point 160 F. was subjected to theductility test and yielded a ductility of 0.020 inch.

Example VI A blend of 80% 133/135 AMP parafiin wax and ceresin wax ofmelting point 185 F. was subjected to the ductility test and yielded aductility of 0.010 inch.

Example VII A blend of 85% 133/135 AMP parafiin Wax and 15%microcrystalline Wax of melting point 160" F. was subjected to theductility test and showed a ductility of 0.020 inch.

Example VIII A blend of 77% 125/ 127 AMP paraffin wax, 20% ceresin waxof melting point 185 F., and 3% liquid propylene polymer of molecularweight 1140 (Staudinger method) was subjected to the ductility test andwas found to have a ductility of 0.033 inch.

Example IX A blend of 77% 125/127 AMP paraffin wax, 20% ceresin wax of185 F. melting point, and 3% of a 1 to 1 mixture of liquid propylenepolymer'of molecular weight 680 (Staudinger method) and solid atacticpropylene polymer of 5800 molecular weight (Staudinger method) wassubjected to the ductility test and found to have a ductility of 0.043inch.

Example X A blend of 78% 125/ 127 AMP parafiin Wax, 20% ceresin wax(M.P. 185 F.) and 2% of a 7 to 1 mixture of liquid propylene polymer ofmolecular weight 740 and solid atactic propylene polymer of 6500molecular weight was subjected to the ductility test and yielded aductility of 0.034 inch.

Example XI A blend of 77% 133/135 AMP parafiin Wax, 20% ceresin wax of amelting point 185 F. and 3% of a 7 to 1 mixture of liquid propylenepolymer of molecular weight 740 and solid atactic propylene polymer of300,000 molecular weight was subjected to the ductility test and yieldeda ductility of 0.044 inch. 4

Example XII A blend of 70% 125/ 127 AMP parafiin wax, 20% ceresin wax ofa melting point of 185 F., and 10% of a' 1 to 1 mixture of liquidpropylene polymer of molecular weight 680 and solid atactic propylenepolymer of 6 5800 molecular weight was subjected to the ductility testand yielded a ductility of 0.060 inch.

Example XIII A blend of .75% 125/ 127 AMP paraflin wax, 20% ceresin waxof a melting point of 185 F., and 5% of a l to 1 mixture of liquidpropylene polymer of molecular weight 680 and solid atactic propylenepolymer of 5800 molecular weight was subjected to the ductility test andyielded a ductility of 0.060 inch.

Example XIV A blend of 77% 133/135 AMP parafiin wax, 20% ofmicrocrystalline wax of melting point 160 F., and 3% of a 7 to 1 mixtureof 31 A.P.I. gravity mineral oil of viscosity at F. of 142 S.U.S. and at210 F. of 42.2 S.U.S. and a solid atactic propylene polymer of 305,000molecular weight was subjected to the ductility test and showed aductility of 0.039 inch.

Example XV A blend of 77% 127 AMP parafiin Wax, 20% microcrystalline Waxof melting point F., and 3% of a 7 to 1 mixture of 31 A.P.I. gravitymineral oil of viscosity at 100 F. of 142 S.U.S. and at 210 F. of 42.2S.U.S. and a solid atactic propylene polymer of 300,000 molecular weightwas subjected to the ductility test and was found to have a ductility of0.052 inch.

Example XVI A blend of 73% 133/135 AMP paraflin wax, 20% ceresin wax ofmelting point F., and 7% of a 1 to 1 mixture of 31 A.P.I. gravitymineral oil of viscosity at 100 F. of 142 S.U.S. and at 210 F. of 42.2S.U.S.

and a solid atactic propylene polymer of 290,000 molecular weight wassubjected to the ductility test and was found to have a ductility of0.050 inch.

Example XVII A blend of 79% 125/ 127 AMP paraflin wax, 20% ceresin Waxof melting point 185 F., and 1% of a 1 to 3 mixture of isotacticpropylene polymer (molecular weight about 200,000-450,000) and liquidpropylene polymer (molecular weight 475) was subjected to the ductilitytest and was found to have a ductility of 0.036 inch.

Example XVIII A blend of 64% 125/127 AMP paraifin wax, 20% ceresin waxof melting point 185 F., 15% microcrystalline wax of melting point 160F. and 1% of a 1 to 1 mixture of isotactic propylene polymer (molecularWeight about 200,000-450,000) and liquid propylene polymer (molecularWeight 475) was subjected to the ductility test and Was found to have aductility of 0.034 inch. I

Example XIX A blend of 64% 125/127 AMP parafiin wax, 20% ceresin wax ofmelting point 185 F., 15 microcrystalline Wax of melting point 160 F.and 1% of a 1 to 3 mixture of isotactic propylene polymer (molecularWeight about 200,000-450,000) and liquid propylene polymer (molecularweight 475) was subjected to the ductility test and was found to have aductility of 0.050 inch.

Example XX Example XXI A blend of 64% 125/127 AMP parafiin wax, 20%ceresin wax of melting point 185 F., and 15% microcrystalline wax ofmeltingpoint 160 F. was subjected to the ductility test and was found tohave a ductility of 0.024 inch. 7 a

We have found that await blend suitable for dipping milk cartons onconventional dipping machines can be prepared by blending liquidpolypropylene with parflin wax and either microcrystalline wax orceresin wax. An improved formulation is obtained by using a mixture ofliquid propylene polymer and solid propylene polymer. Using atacticpolypropylene We have found that ordinary mineral oil can be substitutedfor the liquid polypropylene with satisfactory results. pecteddiscovery. The concentrationof components'niust be about the followingranges:

Broad Preferred Operating Operating Limits Limits Microcrystalline orceresin wax, percent -35 15-25 Paraffin Wax, percent--.; 91-55 84-70Liquid propylene polymer, percent 1-10 1-5 .When liquid propylene resinor alternatively mineral oil is ble'ndedwith 'solid propylene polymerthe limits When isotactic' solid propylene polymer is used theliquidpropylenepolymer must be used and the limits are as follows:

Broad. Preferred Operating Operating Limits Limits Microcrystalline orceresin Wax, percent 5-35 15-25 Parafifin wax, percent 94-55 84-70Liquid propylene polymer and solid propylene polymer, percent (ratiovarying from 1:1 to 1:7 solid to liquid) 1-10 1-5 The concentrationlimits are defined in part by the performance requirements of'the wax'and the coating machines The upper limits of microcrystalline wax'and'liquid polypropylene or mineral oil are dictated by the viscosityrequirements that the blend must not exceed 50 Saybolt Universal secondsat 210 F. in order to permit coating operations atla temperature below200 F. The upper limit of the liquid polypropylene is governed by theamount that the wax'will hold'as Well as the degree of rub-01fresistance desiredi' y The larger the amount of liquid propylenepolymer; the lowerlthe rub-oifresistance. The lower limits aredetermined in all cases by that concentration at which an improvement isnoticeable or a desirable property obtained.

In practice it is generally not necessary or desirable to prepare apolymer which is entirely (100%) isotactic, i.e. stereospecificallyoriented, nor is it necessary or desirable to prepare a polymer which isentirely randomly oriented oratactic; Within the same molecule thereoccur areas (a number of monomer units) which-are atactic and otherswhich are isotactic. The polymerization conditions determinethe relativeamounts of the two typesl Using newly'developed stereospecificcatalysts, it is possible to control thereaction so that there isa'predominance of the isotactic structure within most of the molecules.-A small fraction of the molecules will, however, contain a predominanceof atactic areas. The relative amounts of the atactic and isotacticareas determine the overall properties of the molecule, Therefore, whenpropylene polymer This was a completely unex- 8 is described as atacticor isotactic, it is intended and understood to cover situations in whichthe atactic or isotactic areas of the molecules predominate and notnecessarily a molecule which is either fully randomly oriented or fullyspecifically oriented.

Commercial isotactic propylene polymers are prepared by a solutionpolymerization process using a stereospecific catalyst. At the end ofthe polymerization reaction the isotactic polymer is removed from thereactor as a solid which is not soluble inthe polymerizing solution. Forinstance the polymerizing solution may be an aviation alkylate in whichthe propylene concentration may be about 15% by weight. The catalyst forthe reaction may be titanium trichloride, aluminum triethyl or aluminummonoethyl dichloro or a combination of these or similar materials.Hydrogen at a concentration of about 15 ppm. can be used as a chainstopper. The reaction may be batch or continuous at normal or advancedpressure for a period of about 8-10 hours at a temperature of 150- 200F., the mixture being vigorously agitated during reaction. The reactionproduct is transferred to a solvent bath such as methyl alcohol fromwhich the isotactic propylene polymer precipitates and is filteredtherefrom. This product, when dried, is the isotactic propylene polymer.The polymer remaining in the solvent is separated by evaporation of thesolvent or otherwise. This product, when dried, is the atactic propylenepolymer. Further details of the manufacture of propylene polymer, thecatalyst used or the separation of isotactic and atactic polymer isfound in US. Patent No. 2,874,153 Whichissued February 17, 1959, as wellas in the September 1957 issue of Scientific American which is devotedto the subject of Giant Molecules.

It is understood that minor ingredients-may be added to the wax blend inamounts suflicient to perform a particular function without impairingthe effectiveness of the blend. For instance, well knownoxidation-retarding materials, such as butylated hydroxy toluene, may beadded in fractional amounts such as 0.0005%. Since the wax is maintainedat elevated temperature for long periods of time, oxidation of variouswax components may occur, and hence this may be resisted by adding anoxidationretarding agent such as butylated hydroxy toluene to the blend.

The illustrations of the invention given hereinabove are merely for thepurpose of demonstrating the invention. The only limitations intendedare found in the attached claims.

We claim:

1. A wax composition for coating milk containers and similar articlesconsisting essentially of: about 94-55% by weight of parafiin wax havinga melting point about -150 F., about 5-25 by weight ofmicrocrystallinewax having a melting point about -200 F., and about 1-10% by weight of amixture of liquid propylene polymer and solid propylene polymer, inwhich the solid to liquid ratio is between about 1 to 1 and about 1 to7. i

2. A wax composition for coating milk containers and similararticlesconsisting essentially of: about 84-70% by weight of parafiinwax having a melting point of about 120-150 C., about 5-35 by weight ofmicrocrystalline wax having a melting point of about 150-200" F., andabout 1-5% by weight of a mixture of liquid propylene polymer and solidpropylene polymer in which the solid to liquid ratio is between about 1to 1 and about 1 to 7.

3. Claim 1 in which the solid propylene polymer is an isotactic polymer.

4. Claim 1 in which the solid propylenepolymer is an atactic polymer.

5. Claim 2 in which the solid propylene polymer is an isotactic polymer.

' 6. Claim 2 in which the solid propylene polymer is an atactic polymer.7

- 7. A wax composition for coating milk containers and similar articlesconsisting essentially of: about 94-50% by Weight of paraiiin Wax havinga melting point of about 120-150 F., about 5-35% by weight ofmicrocrystalline wax having a melting point of about 150-200" F., andabout 1-15% by weight of a mixture of liquid propylene polymer and solidatactic propylene polymer in which the solid to liquid ratio is betweenabout 1 to 1 and about 1 to 7.

8. A wax composition for coating milk containers and similar articlesconsisting essentially of: about 84-70% by weight of parafiin wax havinga melting point of about l20-l50 F., about 15-25% by weight ofmicrocrystalline wax having a melting point of about 150-200" F., andabout 1-10% by weight of a mixture of liquid propylene polymer and solidatactic propylene polymer in which the solid to liquid ratio is betweenabout 1 to 1 and about 1 to 7.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Warth: The Chemistry and Technology of Waxes, 2nd edition,pages 502, 503, and 800, Reinhold Publishing Corporation (1956).

1. A WAX COMPOSITION FOR COATING MILK CONTAINERS AND SIMULAR ARTICULESCONSISTING ESSENTIALLY OF: ABOUT 94-55% BY WEIGHT OF PARAFFIN WAX HAVINGA MELTING POINT ABOUT 120-150*F., ABOUT 5-25% BY WEIGHT OGMICROCYSTALLINE WAX HAVING A MELTING POINT ABOUT 150*-200*F., AND ABOUT1-10% BY WEIGHT OF A MIXTURE OF LIQUID PROPYLENE POLYMER AND SOLIDPROPYLENE POLYMER IN WHICH THE SOLIDS TO LIQUID RATIO IS BETWEEN ABOUT 1TO 1 AND ABOUT 1 TO 7.